Rain, Snow, Ice, Dust:
It's All Grist For
NCAR's Cloud Physicists

How do clouds form? That simple meteorological question has a
devilish number of answers. Throw precipitation into the mix, and
things get even more complicated. In spite of the attendant difficulties,
NCAR's cloud physicists have been forging ahead with observational
and theoretical research to define the processes at work in cloud
development.

NCAR became a center of cloud physics research from its earliest days,
when the late scientists Pat Squires and Doyne Sartor arrived in the
1960s. In the 1970s, more experts gathered here for the multiyear
National Hail Research Experiment. After NHRE closed its doors and
U.S. efforts at weather modification dwindled, the discipline
languished. Still, NCAR retained a nucleus of cloud physicists doing
basic research. Now, this group of scientists--one of the discipline's
largest--is finding its work once again fashionable, this time in the
quest for understanding the earth's climate.

Charlie Knight, for one, is glad. "Precipitation development ranks
among the most fundamental and most important areas in
atmospheric science," he says.

Charlie Knight. (Photo by Bob Bumpas.)

Among several dozen cloud physicists spread across several NCAR
divisions, Charlie is the senior member. He joined NCAR in 1962 and
is now a senior scientist in the Mesoscale and Microscale Meteorology
Division (MMM). Charlie's wife and colleague, Nancy, also has been a
long-time MMM researcher.

The books that line the walls of Charlie's office in the Foothills Lab,
including Polymer Chemistry and Crystallography, bear witness to the
multidisciplinary skills needed to study clouds. "I was a geologist in
school," says Charlie. "Then I got into ice. Ice is a crystal, and I had
some expertise in that. Now I'm studying warm rain, which I haven't
worked on before."

The problem now capturing Charlie's attention is how tropical clouds
form raindrops before they have built upward to the freezing level. It's
called warm rain formation, and, says Charlie, it is "one of the classical
problems in cloud physics--in fact, Doyne Sartor used to work on this.
Warm rain occurs much faster than theory can explain. Exactly how
long it should take depends on your theory; how long it does take
depends on your observations. Neither is very well defined."

This summer Charlie heads to Florida for the Small Cumulus
Microphysics Study, officially SCMS, although most of its participants
refer to it as SCUM. The project takes place 3 July to 17 August from a
base about 20 kilometers north of Cape Canaveral, the same area
studied in 1991 for the Convective and Precipitation/Electrification
Experiment (CaPE). NCAR's CP-2 radar will be shipped to its former
CaPE site for its last-ever research expedition. The study also involves
scientists from NCAR; the Universities of Illinois, Chicago, and
Wyoming; the Desert Research Institute (DRI); the New Mexico
Institute of Mining and Technology; and the French government's
meteorological research agency.

The goal of SCMS is to capture the critical few minutes in which a
building cumulus begins to form its tiniest raindrops. On hand to
penetrate the cloud at those moments will be NCAR's C-130,
Wyoming's King Air, and France's Merlin aircraft (which Charlie
describes as "a longer King Air"). It will be the first time both radar and
aircraft have focused on rainfall development in clouds this small.
Though the experiment's spatial scale is modest, the plans are
ambitious.

"All the real action, the important stuff that we don't understand,
happens very early. By the time a cloud down there is about two
kilometers deep, it starts to make rain," explains Charlie. "Once it's
four or five kilometers deep, it hits the freezing level. We have to
have the planes near the radar and ready to respond quickly, we hope
within five minutes." The entire sequence of events, from initial radar
returns to multiple aircraft penetrations, will last no more than a
quarter hour.

What might SCMS uncover? "The biggest question is how the rain
gets started. Another is how much variability there is," Charlie says.
Of particular interest are the size distributions of droplets in each cloud.
These spectra are hard to measure in experiments and even harder to
explain from theory. "They're important for all kinds of things, like
cloud albedo, and they're quite mysterious. There are a lot of bimodal
spectra [peaking at two different sizes], and in general there are more
small droplets than you would expect from simple condensation
theory.

"It's very complicated, very variable. Once in a while, somebody calls
me and asks what the droplet sizes are in cumulus. There's no simple
answer to that."

As a visitor to Charlie's office might surmise from his potpourri of
books, papers, and lab supplies, he always has "three or four things
going on at one time." For instance, his crystallographic interests have
led him to study the molecular structure that allows fish to survive in
freezing seawater. Charlie is now pursuing work on dendritic
snowflake growth with DRI's John Hallett. As for the SCMS data to
come, "it'll be analyzed for at least five years. If all goes well, I'd love to
go to Hawaii and do it all over again in a more maritime setting."
Why? Among other reasons, the mid-Pacific air of Hawaii has only
one-tenth the number of cloud condensation nuclei (CCN) that are
present over Florida.

The Heart of the Matter

CCN are at the root of cloud and precipitation formation. They are also
a current interest of Al Cooper, head of MMM's physical meteorology
group. Al offers two somewhat tongue-in-cheek definitions for his
group's focus: "Physical meteorology is everything that's not dynamic
meteorology. We like to think the dynamicists have the Navier-Stokes
equation and we have everything else."

Al Cooper. (Photo by Bob Bumpas.)

CCN on the order of 0.01 millimeter (one micrometer) in diameter or
larger are the most active in cloud formation. The largest ones, such as
sea salt or desert dust, are important in the formation of rain. Al is
looking at the impact of these large particles in SCMS. "There have
been very interesting preliminary results in South Africa where clouds
have been seeded with CCN on the order of one micrometer." In the
past, weather modification studies have tried to use CCN as large as 20
micrometers as individual raindrop embryos. The South African
work, says Al, is instead trying to stimulate the collision-coalescence
process and nudge the entire droplet distribution spectrum toward
larger drops. The collision-coalescence process refers to the mechanism
by which droplets of different sizes fall at differing rates, causing
collisions (just as might happen on an interstate if some cars moved at
80 kilometers per hour and others at 130). These collisions tend to
result in larger, coalesced drops that fall even faster, provoking more
collisions in a cascading process that culminates in rain.

Al will be examining the droplet distribution in SCMS with an eye
toward the presence and evolution of giant particles and resulting
droplets. Eventually, he'd like to see better models of the collision-
coalescence process. "Do we really understand this well enough to
calculate what will happen? I think we understand the basic process,
but I'm not sure we have the collisions right." There are several
difficulties:

Collision efficiencies (the amount of coalescence from a given
number of collisions) are hard to calculate and verify.

Electrical effects change the collision efficiencies.

Turbulence is hard to account for.

The effects when relatively dry air is entrained into the edges of a
cloud are very poorly understood.

One thing is certain: the earth's atmosphere is loaded with CCN.
Currently observed levels are from 50 to 200 CCN per cubic centimeter
over the pristine ocean (that is, away from large land sources) and from
500 to 1,000 CCN over land. Some counts in the thousands are
observed downwind from urban areas. Says Al, "Worldwide, biomass
burning could be the largest source of CCN, especially the burning of
grasslands and forests in the tropics. We probably have more CCN
now than ever before."

Ironically, the growth in CCN concentrations might work to reduce
global precipitation. As more CCN compete for a given amount of
water vapor, the range in droplet sizes is reduced (the droplets are
smaller and greater in number) and fewer collisions occur. The sheer
presence of more aerosols also helps to obscure incoming solar
radiation, cooling the atmosphere and making it more stable, a process
Al says was "very dramatic" during the Kuwait oil fires studied by
NCAR in 1991. Thankfully, the effects were primarily local.

The connections between microscale physics and global climate are the
impetus for a new CCN/IN (ice nuclei) counter being developed by Al
and Larry Radke, director of NCAR's Research Aviation Facility.
"We're calling it a CCN/IN counter, but it's primarily designed to
count the nuclei involved in cirrus and contrail formation. If it does
all of this, it'll be a great success. If it does only some, it'll still be
useful." The instrument will be flown at high altitudes worldwide,
first on NASA's DC-8 and eventually on NCAR's WB-57. Meanwhile,
Al's group is looking at new ways to use such data. For instance,
through a joint appointment between MMM and the Atmospheric
Chemistry Division, Mary Barth is studying the effect of clouds on
tropospheric chemistry.

Like Charlie, Al is glad to see the resurgence of interest in cloud physics
for global change research after its 1960s-70s role in cloud seeding.
"This is really not that great a change. Both are questions of weather
modification--one intentional and the other unintentional."

Tracing the Life of Ice

Cloud watchers long have marveled at the sublime streaks of classic
cirrus clouds, often referred to as mare's tails. MMM's Andy
Heymsfield is studying the mechanics behind the beauty of cirrus.

Andy Heymsfield. (Photo by Bob Bumpas.)

The goal isn't to deconstruct the experience of cloud watching but to
understand how cirrus evolve by "seeding" themselves and clouds
below. Andy recently collaborated with Sam Oltmans (NOAA
Aeronomy Laboratory) and MMM associate scientists Larry
Miloshevich and Steve Aulenbach to evaluate the humidity profiles
and crystal structures throughout a cirrus cloud that formed near
Boulder on 10 November 1994. The study used a cryogenic (ice-based)
hygrometer to measure relative humidity at temperatures as low as
-60¡ C. A separate instrument package aboard the same radiosonde
captured crystals at various levels, drenching and preserving each
crystal in a quick-drying liquid plastic. With this technique, says Andy,
"you don't have the problems you have with crystal breakup when
aircraft are moving at 250 kilometers an hour trying to take samples.
The other advantage is that you get a vertical profile instead of a pass
through a cloud at one level."

Documenting cirrus is crucial to understanding clouds' effect on global
climate. Andy has been at the forefront of the effort to clarify the role
of cirrus on global radiation budgets, largely through his participation
as a principal investigator in the First ISCCP Regional Experiment
(FIRE) and the Central Equatorial Pacific Experiment (CEPEX). ISCCP is
the ongoing International Satellite Cloud Climatology Project and
CEPEX was a 1992 study of the radiation budget and microphysics of the
tropical central Pacific.
Satellites are particularly adept at mapping global cirrus distributions.
Andy is working to compare research aircraft data to satellite-based
water-vapor data in order to calibrate the latter. "This could give us
cirrus height, optical depth, and particle size information."

Another focus of Andy's work has been "finding out under what
conditions cirrus crystals nucleate." Their formation depends on the
saturation vapor pressure of ice. Cloud droplets form when the
relative humidity exceeds 100%, a state called supersaturation with
respect to water. But for very cold air at humidities just under 100%,
the air is unsaturated with respect to water but highly supersaturated
with respect to ice, allowing cirrus to form. A climate model thus
needs to keep close tabs on temperature and relative humidity in order
to treat cirrus formation accurately.

The picture becomes even more complex when ingredients such as
sulfuric acid come into play, says Andy. Solutions of water and sulfuric
acid are very concentrated at humidities close to the saturation point
with respect to ice, and these particles cannot freeze. But as the
humidity increases, the solutions become more dilute (due to the
added moisture) and freezing becomes more likely.

Andy has joined a number of other NCAR researchers in a modeling
and observational study on the effect of cloud ice on precipitation
development in the Southwest. The Arizona Project is now wrapping
up two months of field work centered on winter storms in the
mountainous area between Phoenix and Flagstaff. The project
examined high-level mountain wave clouds formed as frontal systems
crossed the Mingus Mountain region. These clouds deposit ice crystals
into lower-level clouds formed by upslope winds that ascend the
Mogollon Rim, which extends southeast from Flagstaff. Later this year,
a mesoscale model developed by MMM's Terry Clark will be coupled
with an in-depth microphysical model to test how cloud seeding in the
region might enhance wintertime precipitation and help to build up
Arizona's water storage.

The experiment is building on a number of findings from the
multiyear Winter Icing and Storms Project (WISP), in which scientists
from NCAR's Research Applications Program have observed and
modeled ice and supercooled water in clouds near the Front Range
since 1990. The Arizona Project's principal investigator is Roelof
Bruintjes (MMM/RAP) .

Next Stop: The Stratosphere

Some cloud physics questions range even higher than cirrus clouds.
Over the past decade, the Antarctic's greatly depleted "hole" of
stratospheric ozone has been documented, along with lesser but still
significant ozone depletions at other latitudes. The crux of the
argument for ozone's human-induced depletion has been an
interaction between chlorofluorocarbons (CFCs), polar stratospheric
clouds, and sunlight. Now NCAR has developed an instrument to
help view this and other processes at work on the microscale for the
first time.

Jim Dye (MMM) and Darrel Baumgardner and Bruce Gandrud (both of
ATD) combined forces with other staff from their divisions to create
the multiangle aerosol spectrometer probe (MASP). Winner of the
1994 Technology Advancement Award (see the December 1994 Staff
Notes Monthly), MASP promises to shed light on the ozone depletion
story, as well as on a more recent development in cloud modeling
that's just as sobering in its own way.

Darrel Baumgardner. (Photo by Bob Bumpas.)

"This instrument's advantage," says Darrel, "is that, in addition to
measuring the size and concentration of particles, we can derive
information on their optical properties. If we know something about
their optical properties, we can then deduce something about their
chemical compositions." MASP trains a laser beam on a particle at
several angles. The particle's refractive index can be derived from the
amount of light that is redirected or refracted as it passes through the
particle.

The first flights of MASP took place last year aboard NASA's ER-2 and
went "quite well," says Darrel, with some interesting results already:
"The jury's still out, but I think we're going to find some surprises."
The instrument was deployed four times from Christchurch, New
Zealand, between March and October 1994 to document the ozone hole
and its causative factors. A trek to the Arctic aboard NASA's DC-8 will
follow next year. Darrel awaits the findings with interest. "Chlorine is
released during reactions on the surfaces of PSC [polar stratospheric
cloud] particles that form in the polar winter night. The composition
of these PSCs is still in question, but the MASP measurements of these
particles hopefully will help to clarify theoretical speculations."

MASP may also prove useful in addressing a newly discovered
dilemma in cloud modeling. A study published in Science on 27
January reveals that the global energy budgets normally used in climate
models appear to seriously underestimate the percentage of energy
absorbed by clouds. NCAR's Jeff Kiehl is one of the coauthors. The
paper reports on a comparison between satellite- and ground-based
observations that found clouds absorbing more than 25 watts per
square meter of energy, rather than the 6 watts predicted by theory.

"Where are those extra watts per meter going? That's where the big
mystery is now," says Darrel. He thinks that haze droplets not yet large
enough to count as full-fledged cloud droplets may be part of the
answer, absorbing or scattering the radiation now unaccounted for. "In
cloud measurements, it's typical to ignore droplet sizes below a certain
level." MASP will measure particles as small as 0.3 micrometers,
which "could be an important size for this kind of scattering."

Darrel knows about unexpected findings in cloud physics. On an
expedition off the Florida coast in November 1993, he and King Air
pilot Mike Heiting were unnerved by an intense rain gush that ruined
one water-sensing probe and damaged another.

The probes, both about three centimeters long and one to two
millimeters wide, were inundated by "an incredible mass of water,"
recalls Darrel. "The Russian probe truly was trashed--the water abraded
the probe away." The other probe, which measures liquid water
content by monitoring the amount of heat needed to evaporate the
water, responded too slowly to the very brief gush and burnt itself out
by pumping too much heat to the probe after the gush was over.

"The audio inside the cabin was amazing. It sounded like a giant
'splat.' It makes you ask the question, 'What's going on that could
cause such a gush of rain?' It says that there's some incredible
variability. Every time we go out, we discover something that tells us
just how diverse the atmosphere is." --BH